In the fields where high safety is required, it is required to monitor a function normally operating during its operation. Therefore, the function is duplexed into an operation system and a safety system on a function-by-function basis, such as a module-by-module basis. The operation is monitored by comparing the outputs of those with each other. In this method, the combination circuit except the registers is also needed to be duplexed entirely, which causes the circuit scale to be twice as large or more.
Moreover, there can be a case where common cause failure (for example, occurrence of delay due to temperature variation) arises in the operation system and in the safety system. In such a case, there can be a case where the outputs coincide with each other in the operation system and in the safety system, which causes a possibility that the failure cannot be detected.